[QE-users] Large Difference in HOMO/LUMO Levels Between SCF and NSCF for Slab Calculation

[Ms. Chandrika K.]

I am studying a material that is an insulator in its bulk form. When I convert the system into a slab geometry and run SCF/NSCF calculations in Quantum ESPRESSO, I see unexpected behavior in the electronic structure (large shifts in HOMO/LUMO levels ).
With Occupations =Fixed
    Highest occupied and Lowest unoccupied level (ev): -0.1520 1.5853 from scf.out.
    Highest occupied, lowest unoccupied level (ev): 5.8411 11.1794 from nscf.out.

With Occupations="smearing"

           degauss=0.02

           smearing="gaussian"

I am getting      the Fermi energy is     0.7314 ev----nscf

     (compare with:     0.7311 eV, computed in scf)
Since the material is insulating in bulk but behaves differently when modeled as a slab, should I be using a different method or setting for slab calculations? How do we determine occupations parameter in such cases?

Chandrika Yadav K
Research Scholar
NanoElectronics Laboratory
Amrita Viswa Vidyapeetham
Bangalore
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